1. Field of the Invention
Aspects of the present invention relate generally to generation and maintenance of electromagnetic fields, and more particularly to a system and method of generating and confining magnetic fields that are uniform and controllable in magnitude and direction.
2. Description of Related Art
In many situations, it may be useful to apply a uniform magnetic field over some region of space, for example, to perform certain tests or to obtain measurements with respect to operation of an electronic device containing a magnetic component. A common requirement for systems employing magnetic fields for testing or measuring purposes is that the applied field be uniform in magnitude and direction over the entire expanse of the working region; in many applications, it may be desirable that the field also be controllable in magnitude and direction. In some instances, it is desirable that the magnetic field be nonexistent outside the working region, i.e., the boundaries of the working region define the extent of the magnetic field. Conventional systems generally fail to satisfy these and other requirements.
Magnetic fields may be generated, for example, using current conducting coils. Some traditional attempts at generating nearly uniform magnetic fields have employed so-called “Helmholtz coils,” which are operative to superpose the fields from a pair of picture-frame coils. In practice, field uniformity at the center is best when the coils are separated by a distance approximately equal to the coil radius. Using Helmholtz coils, a reasonably uniform field may be approximated within a central region of the coil pair; the attainable field strength is typically less than 100 Oe. In that regard, Helmholtz coils are useful only for generating fields of low magnitude within an unobstructed environment. Bi-axial fields, which also allow for control of field direction, may be generated using two orthogonal pairs of Helmholtz coils. Compared to their large size, such Helmholtz systems can provide only relatively low field magnitudes as noted above. To generate fields of larger magnitude using traditional technologies, it is generally necessary to sacrifice the degree of field uniformity provided by Helmholtz coils; some systems employ electromagnets in which magnetic flux is channeled through a low-reluctance yoke to a pair of opposed pole pieces. While such electromagnets can be implemented to provide fields of much larger magnitudes than Helmholtz coils, electromagnets typically suffer from significant inefficiencies, and generally do not provide the same degree of field uniformity as is attainable with Helmholtz coils.
FIG. 1A is a simplified diagram illustrating a magnetic field generated by a prior art uni-axial electromagnet system, and FIG. 1B is a simplified diagram illustrating a magnetic field generated by a prior art bi-axial electromagnet system. The illustrations in FIGS. 1A and 1B depict, for each system, the directional configuration of a magnetic field generated between pole pieces of the respective magnet system, as well as the associated distribution of field magnitude. The illustrations also indicate a “working region” in which field magnitude and direction are sufficiently uniform to satisfy typical testing or measurement applications performed by such systems.
FIG. 1A illustrates characteristic degradation of field uniformity in a uni-axial electromagnet as caused by the field bulging from a (central) longitudinal x-axis between a pair of pole pieces 110 and 120. The lack of field uniformity generally limits the useable working region to be substantially smaller than the space between pole pieces 110, 120. This adversely affects size, power consumption, frequency of operation, and manufacturing cost of the system employed to generate the field.
Referring now to FIG. 1B, bi-axial fields may be generated with electromagnets by orthogonally superposing the fields from two uni-axial arrangements. This illustration shows the configuration of a magnetic field generated along the x-axis by a first uni-axial arrangement; a second, substantially identical uni-axial arrangement rotated by 90 degrees relative to the first arrangement, generates a second magnetic field component along the y-axis. In each uni-axial arrangement, magnetic flux is channeled through a low-reluctance yoke to a pair pole pieces (110A and 120A, on the one hand, and 110B and 120B, on the other hand). In this case, field uniformity is further impaired because the field, Hx, from one uni-axial system comprising pole pieces 110A and 120A is shunted by the pole pieces 110B and 120B from the other uni-axial system as indicated in the drawing figure. As a result, the useable working region is typically very small as compared to the distance between pole pieces. Power consumption, frequency of operation, and manufacturing cost are adversely affected in bi-axial systems worse than with typical uni-axial systems.